1. How long are you planning to stay? I assume the whole year, but who knows? You said on one hand that you will join him, and on the other hand that you will be traveling with your mom.
As for car seats and strollers, I'd say visit a secondhand or recycle shop here. VERY good inspection of goods before they are resold.
2. Ask your airline.
3. Well, unless you plan to bring your baby naked, yes. The babe will obviously grow, and you can buy anything you want here. It's not deepest, darkest Africa where babies crawl around nude.
4. Sorry. No advice here. You'll have to shop around yourself. This is the negative side of a married person on JET.
5. First advice: relax. Japanese people don't expect foreigners to even have the ability to learn Japanese. Second: they are the most polite people in the world. Third: get a Berlitz book (pocket size) and start learning the basics (please, thank you, where is the XZ?) Learn to read katakana because many items in supermarkets will have such labels, and they are often used for English words. (There is also English on some labels, but don't count on it.) Fourth, as soon as you find out your husband's location here, learn as much as you can about it, and use the links for discussion forums at
www.bigdaikon.com just to locate any people in similar situations.